The short answer
This small MRI study found less fat within thigh muscles with semaglutide. It did not establish a clear difference in thigh muscle volume or prove that muscle function improves.
Who was studied?
Of 1,407 STEP UP participants, 55 had valid baseline scans for this analysis. Researchers pooled 49 people assigned to semaglutide 7.2 or 2.4 mg and compared them with six receiving placebo over 72 weeks.
Estimated difference; confidence interval includes zero
Difference versus placebo
−1.1 litres
95% confidence interval
−2.3 to +0.1 litres
Participants
49 semaglutide; 6 placebo
Original visual based on the study.
| Group or measure | Result |
|---|---|
| Difference versus placebo | −1.1 litres |
| 95% confidence interval | −2.3 to +0.1 litres |
| Participants | 49 semaglutide; 6 placebo |
What changed?
Compared with placebo, the estimated change in thigh muscle volume was 1.1 litres lower with semaglutide. Its confidence interval ranged from 2.3 litres lower to 0.1 litres higher, so it included no difference.
Fat infiltration within thigh muscle fell by 0.92 percentage points more with semaglutide. This measures a different feature from muscle size.
Both groups averaged 13 chair rises in 30 seconds initially and 15 at week 72.
The useful takeaway
Muscle size, fat within muscle and physical performance are separate measurements. With only six placebo participants, these findings are uncertain. They do not settle what happens to muscle in everyone using semaglutide.
Sources
Hjelmesæth and colleagues, Diabetes, Obesity and Metabolism, 2026.